The present invention is directed to a charge amplifier circuit comprising an operational amplifier (op amp) having a signal output that is fed back to a signal input of the op amp via a feedback element having a capacitor with a finite resistance.
Numerous charge amplifier circuits of this type are known and have significantly contributed to the current, broad application of piezo-electric metrology since deficiencies inherent in electrometer amplifiers were thus capable of being eliminated. The operational amplifier of such a circuit generally is constructed as a direct-current amplifier having optimally high, internal voltage amplification, whereby the polarity of the output voltage is opposite that of the input voltage and whereby an optimally high input resistance is realized. The output of the amplifier is capacitatively negatively fed back to an input of the op amp by means of a highly insulating capacitor (corresponding to a parallel RC element). As a result, the input impedance is practically eliminated but the high isolating resistance at the input is preserved.
Assuming ideal conditions are present, it follows that no voltage arises via transducers, cables and the like connected via the capacitance, so that the isolating resistor in the input circuit is not critical and the input capacitance has no influence on the output voltage. Furthermore, because the output voltage is directly proportional to the charge output by means of a transducer or the like, is directly proportional to the actual measured quantity to be identified, and is inversely proportional to the feedback capacitance, arbitrarily graduated measuring ranges are capable of being achieved in a simple way by connecting corresponding capacitance values.
Taking actual conditions into consideration (the internal amplification of the operational amplifier unit is not infinite but limited to values in the range from 10.sup.5 through 10.sup.7, only a maximum of about 100 T ohms [i.e., 100 Tera (10.sup.12) ohms ] isolating resistances are possible both at the amplifier input as well as for the feedback capacitors; there are no input stages that exhibit absolutely no leakage current; etc.), certain disadvantages nonetheless derive in view of the measuring method or, respectively, of the structure of the circuit, these being felt in a particularly disturbing fashion especially in conjunction with precise (or sensitive) measurements or quasi-statistical measurements. One of the most critical of these disadvantages is based on the fact that the RC element (whether the resistance (R) is intentionally defined by an additional, corresponding resistor that lies parallel to the capacitor or, on the other hand, is only defined by the internal resistance of the capacitor is inconsequential) serving for the capacitative feedback of the operational amplifier unit establishes a break-over frequency in the transmission behavior of the charge amplifier circuit. The circuit then is placed under a operating frequency range within a certain safety margin in the interest of maintaining an optimal linear transmission behavior. In view thereof, extremely low frequencies are eliminated from consideration anywhere in the standard measurements of such arrangements.
The necessity of making the product R*C of the operational amplifier feedback optimally high thus directly derives from the foregoing. Since an increase in the size of the feedback capacitor reduces the charge sensitivity of the circuit in practically inversely proportional fashion, the only possibility that remains is to make the resistance in the feedback circuit as high as possible. According to the present state of the art, as mentioned, values of resistance of a maximum of about 100 T ohm can be realized in this context--but this only under the most favorable conditions and given the assistance of careful, manual re-working of the circuit formatting. Apart from the fact that these high resistance values cannot be reproduced without a great amount of effort or, are not durable, disadvantages such as higher and higher resistance noise given higher values of resistance as well as dependency of the charge voltage transmission of the operational amplifier unit on the relationship between feedback resistance and input resistance also derive. See, for example, "Tichy, Gautschi, Piezoelektrische Messtechnik, Springer-Verlag, Berlin, Heidelberg, N.Y., 1890", particularly Chapter 12.